This invention is concerned with improvements in or relating to the separation of solids from liquids by screening. Often it is desired to separate one or more solids from one or more liquids where beforehand each solid existed in suspension in each liquid. The purposes of the separation are the recovery of either the solids or the liquids or both for further processing or for the release of one or the other into the environment. In this specification the term `suspension` will be used with reference to the admixture of one or more solids and one or more liquids prior to screening and the terms `solids` and `liquid` will be used with reference to the products of the screening process, though it is realized that some liquid remains with the solids and some solids, or a sufficiently small particle size, remain in suspension in the liquid after screening.
Many types of apparatus and methods have been used for separating solids from liquids by screening. Static screens have often been used, the suspension flowing over a weir and down one or more inclined screens under the influence of gravity. The solids move down the screen to be collected at the bottom while the liquid drains through to be collected or discharged below the screen. However, static screens, while allowing continuous screening, handle only relative low flow rates and thus a large screening area is required to handle large flow rates. Furthermore, since the screens must slope downwardly, and because a relatively long length of screen is required for effective separation, the head of suspension required at the top of the screen is relatively large.
Rotary screens comprising a hollow rotatable drum having a perforated screening wall and having means for the delivery of the suspension to the interior of the drum are an improvement for by their rotation they continually present fresh screening surface to the suspension, this allowing larger flow rates to be handled than with static screens and they can be operated with a lower head of suspension than can equivalent static screens. One such rotary screen is described in U.S. Pat. No. 2,661,201. In this case, the wall of the drum has a plurality of relatively large circular perforations. This type of screen is suitable where the solids are in the form of relatively large particles but is not satisfactory for handling solids in the form of fine particles or fibres or globules, for the solids, depending on their size and composition, can either pass through the perforations with the liquid or clog the perforations. U.S. Pat. No. 3,961,605 discloses a rotary screen where the wall of the drum is made of a wedge shaped wire wound helically about support rods. The use of wedge wire reduces clogging but the screening efficiency is impaired by having, on rotation of the screen, the major component of the movement of the suspension relative to the screen substantially parallel to the coils of the screen wire. It has been found that screening efficiency, where screen wire is used, is improved by having the major component of the movement of the suspension in a direction substantially normal to the screen wire to produce a shearing effect. It was with this particular effect and the aforementioned disadvantages in mind that the present invention was devised.
The apparatus of the invention has applications in the screening of solids from waste waters and sewage liquors and in the thickening of slurries and pulses for example, amongst many another applications in industries such as freezing works, tanneries, pulp and paper mills, canneries, fish and meat processing and sewage treatment works.